xDSL is a high speed data transmission technology for transmission over telephone twist pair (Unshielded Twist Pair, UTP). In addition to digital subscriber line (DSL) of baseband transmission such as IDSL and SHDSL, xDSL of passband transmission enables xDSL and traditional telephone service (POTS) to coexist in a same twisted pair by using frequency-division multiplexing technology. The xDSL occupies high band and the POTS occupies baseband below 4 KHz, POTS signal and xDSL signal are separated by a splitter. The xDSL of passband transmission uses discrete multi-tone modulation (DMT). A system providing multiplex xDSL access is referred to as a DSL access multiplexer (DSLAM), and a system reference model thereof is shown in FIG. 1.
A rate, for example, a line rate of downlink 2M, is determined when xDSL is on; or a line rate, for example, 4M, is determined according to a line status during training the xDSL transceiver. Once the rate is determined, the line rate keeps constant during practical operation, regardless of the rate of service data flow, which will be inflexible when bit error rate needs to be lowered.
Technically, in order to maintain constant line rate, the transmission power on line should not vary all the time. For example, the line rate and total transmission power that are determined during line training are R0 and P0 respectively. Assume that the information that can be transmitted when a line rate is sufficiently used is I0, then
      I    ⁢                  ⁢    P    ⁢                  ⁢    R    =            I      0              P      0      is the information power ratio of the line transmission. However, in case of a line rate of R0, the information transmitted I is less than I0, i.e., I<I0, because the total signal transmission power P0 needs to be kept constant in order to maintain the line rate R0. In this case,
      I    ⁢                  ⁢    P    ⁢                  ⁢    R    =      I          P      0      will be small. In other words, less information is transmitted with the same power, which results in a waste of supply power.